Recently, there has been increased interest in the development of anion exchange membrane fuel cells (AEMFCs). The fundamental difference between AEMFCs and the more widely studied proton exchange membrane fuel cells is that the former operate at high pH thus requiring the membrane to conduct hydroxide ions from the cathode to the anode. The key advantage of operating a fuel cell under alkaline conditions is the potential to forgo noble metal catalysts due to the low overpotentials associated with many electrochemical reactions at high pH. The improved electrokinetics also allow for the possible use of high energy density fuels such as ethanol which is also a renewable resource as it can be produced directly by fermentation of biomass.
A major challenge in the development of AEMFCs is the need for an anion exchange membrane (AEM) that is chemically stable under the conditions within an AEMFC. AEMs are typically made with polymers that have pendant cationic groups. By far the most commonly reported cationic group is the benzyl trimethylammonium (BTMA) cation. AEMs have been prepared with BTMA cations attached to polymer backbones such as poly(phenylene), poly(tetrafluoroethene-co-hexafluoropropylene), poly(phenylene oxide), poly(ether-imide), poly(arylene ether sulfone), and poly(ether ether ketone).
Many of these BTMA-containing membranes are reported to have good chemical stability. For example, the ion exchange capacity of a radiation-grafted perfluorinated AEM with BTMA cations was shown to decrease by less than 5% after a 233-hour fuel cell test at 50° C. Another study of the degradation mechanisms of tetraalkylammonium compounds concluded that maintaining hydration around the cations is critical to stability and that, under the correct conditions, such cations possess reasonable stability at temperatures above 60° C. Despite reports such as this, BTMA cations are generally considered to have insufficient stability for long-term use in AEMFCs. Thus the investigation of cationic groups with improved chemical stability is of paramount importance to the development of AEMFCs.
One relatively early study of cation stabilities found that quaternized 4,4′-diazobicyclo-[2.2.2]-octane cations had improved stability to alkaline conditions when compared to BTMA cations. Another approach to preparing more stable cations is to reduce susceptibility to nucleophilic attack by using resonance-stabilized cations such as guanidinium or imidazolium groups. Other reports have included the use of coordinated metal cations or phosphonium cations with bulky electron-donating substituents to both sterically protect the ion from nucleophilic attack and to lessen the charge density on the phosphorous atom. Additionally, it has been reported that attachment of quaternary ammonium groups to the polymer backbone via an alkylene spacer of >3 carbon atoms can lead to improved chemical stability. Attachment of imidazolium and guanidinium groups with alkylene spacers have also been reported.
AEMFCs also require a polymeric binder, or ionomer, to minimize interfacial resistance between the membrane and the electrodes. Typically, the ionomer is dissolved or suspended in a solvent which is then combined with the catalyst. The resulting catalyst ink is then painted or sprayed onto either the membrane or the gas diffusion layer prior to assembly of the cell. The role of the ionomer is to maximize the transport of ions, fuel, oxygen, and water within the electrodes. This is distinctly different from the role of the membrane, which is to block the passage of fuel and oxygen between the anode and the cathode while allowing the passage of ions and water. Despite these different roles, very polymers have been designed specifically to function as ionomers in AEMFCs and it is common practice to use the same polymer as both the membrane and the ionomer. This lack of optimized ionomers results in increased interfacial resistance between the electrodes and the membrane and a decrease in the efficiency and power output of the fuel cell.
What is needed is a polymer that is designed to be used as an ionomer in AEMFC electrodes. Such a polymer would have mechanical and chemical integrity to enable it to remain in place, unchanged during fuel cell operation. It would also have sufficient flexibility to enable the rapid permeation of a gas or liquid fuel to maximize the rate of reaction on the catalyst surface. It would also have the appropriate level of hydrophilicity to enable the movement of water within the electrodes and to avoid the overhydration condition known as flooding.
What is also needed is a binder that is chemically similar to the membrane and which is more permeable to oxygen and the fuel of interest than the membrane polymer. The ionomer should have water swelling properties that are similar to those of the membrane in order to avoid delamination of the electrodes during hydration/dehydration cycles in the fuel cell. The ionomer must be soluble or form a suspension in water, low molecular weight alcohols, or some combination of those. Also, the ionomer needs to have a chemical stability at high pH that is at least as good as the membrane because very thin layers of the ionomer will be subjected to high fluxes of hydroxide ions in the fuel cell during operation.
What is also needed is an anion exchange membrane with high ion conductivity and good chemical stability at high pH. The membrane should act as a barrier to the fuel of interest and to oxygen. Ideally, the membrane should have low water swelling, although low water content tends to reduce the conductivity, so these two properties must be balanced according to the needs of the specific cell.